AGE OF THE GEM STOCKS, COEUR D'ALENE DISTRICT, IDAHO

The Gem stocks are monzonite intrusive bodies, emplaced into a thick sequence of Proterozoic clastic strata of the Belt Supergroup in the Coeur d'Alene mining district of northern Idaho. SHRIMP U-Pb, Ar-Ar, and Rb-Sr ages on samples of the monzonite, associated intrusive rocks, and ore-bearing veins address the timing of their emplacement and thermal history. U-Pb ages of magmatic zircon from samples of the South Gem stock by single-grain, ion microprobe analysis on the USGS/Stanford University SHRIMP define ages with modes at about 122 Ma, 116 Ma, and 113 Ma, with a mean of 118±4 Ma. Dispersion of ages of zircon rims is substantially larger than analytical error and their distribution on a concordia plot suggests both Pb loss and minor, unresolved inheritance. SHRIMP ages on sphene from the same samples range from 118 Ma to 94 Ma with a mean of 105±3 Ma. Ar-Ar age spectra for hornblende coexisting with the sphene and zircon in these samples define irregular patterns typical of excess Ar, with elevated ages of low- and high-temperature increments up to 400 Ma and lower values approaching the intrusive age in intermediate temperatures. Minima of these age spectra indicate a maximum age of 113±1 Ma for hornblende closure in these shallow intrusive stocks.

Ar-Ar ages of muscovite and Rb-Sr muscovite, K-feldspar, whole-rock isochrons from a pegmatite dike in a wall-rock pendant in the South Gem stock are concordant at about 110 Ma. Alkali feldspar from a sphalerite-bearing vein in the Rex mine within the margin of the South Gem stock yields an excess Ar pattern with ages above 2 Ga in low- and high-temperature increments, but decreases to about 108 Ma in its 925°C step. The feldspar not only indicates a maximum age in agreement with the pegmatite ages, but its 87Sr/86Sr initial ratio >0.800, radiogenic Pb isotopes, and coexistence with sphalerite establishes a maximum age for these ore-bearing fluids.

K-feldspars from the South Gem stock also reveal an Early Tertiary thermal event affecting the Gem stocks and adjacent wall rocks. Ar-Ar age spectra define a 40-50% resetting of the Ar system of K-feldspar at 54 to 51 Ma. These ages coincide with those of nearby mafic dikes and with movement on the right-lateral Osburn and the low-angle Dobson Pass faults, the latter being responsible for tectonic unroofing of the Gem stocks.